A hollow fibre membrane is a capillary whose wall functions as a semipermeable membrane. Hollow fibre membranes cover a wide range of separation problems with a specific membrane, of suitable membrane characteristics, being most suitable for each problem. The membrane characteristics need to be established to ascertain the optimal membrane for specific application. Such characterisation methods may be different depending upon the mechanism of specific separation, as will become clear from the description provided below.
The membranes for use in achieving mass transfer from a first fluid to a second fluid are unusual in membrane processes, in that the membranes are essentially nonselective. The selectivity for the species transferred is achieved by careful selection of the first fluid and the second fluid. Moreover, transport (mass transfer) of a species or component across the membrane are generally diffusion driven by an imposed concentration gradient (partial pressure gradient) of the component between the inside and the outside of the hollow fibre, and/or diffusion driven by chemical absorption.
The production of hollow fibres, used for conventional membrane processes such as reverse osmosis, ultrafiltration, microfiltration, dialysis, gas separation, pervaporation etc., are well known in the prior art (e.g. U.S. Pat. Nos. 3,899,309, 4,293,418, 4,430,219, 4,690,873, 4,717,394, 4,758,341, 4,781,834, 4,911,846, 4,961,760, 5,026,479, EP-A 0446947, Cabasso, I. et al. "Hollow Fibre membranes", Kirk-Othmer: Encycl. of Chem. Tech., Vol 12, 3. ed., pp 492-517 (1980)).
In the production of hollow fibre membranes, the critical physical parameters are generally considered to be the diameter of the hollow fibre, the wall thickness, the pore size, and the porosity of the membrane.
The basic wall morphology of the hollow fibres has been selected to obtain desired hydraulic permeability and desired mechanical properties. The key concerns for membrane characteristics for pressure driven processes such as reverse osmosis, ultrafiltration, microfiltration, gas separation etc. have been hydraulic permeability (i.e. convective permeability) and rejection characteristics of the membranes. Characterization of suitable hollow fibre membranes for such applications is well described in the prior art (e.g. Kesting, R. E., "Synthetic Polymeric membranes", McGraw-Hill Book Company, 1971; Mulder, M., "Basic principles of membrane technology", Kluwer Academic Publ., Dordrect, 1991). Generally, structural parameters as thickness, pore size and porosity of the membrane are combined in a hydraulic permeability constant, which is measured directly by a suitable method. Such measurement of hydraulic permeability, when used in combination with retention characteristics, such as the molecule cut off value, specifies the transport characteristics of the membrane.
When dealing with mass transfer through hollow fiber membranes with gas-containing pores, the same concerns have guided the selection of suitable membranes. Thus, e.g., EP 0374873 discloses the use of hollow fibre membranes for absorption/desorption where the hydraulic permeability has been specified for selecting the membrane.